Internet Protocol TV (IPTV) is a term used when delivering broadcasted TV services over an IP network, typically a broadband access network. Currently the predominant IPTV service is Broadcast TV, in which the normal non-IPTV channels, as well as additional channels with low penetration are transmitted over the broadband network from the super head-end down to the end-user's set top box (STB). In order to minimize the bandwidth required for these transmissions it is desirable to use multicast techniques through the network. When the user switches channel the STB then sends out an Internet Group Management Protocol (IGMP) message to leave the current multicast channel and join a new multicast channel. In IGMP version 3 this is done in the same message while in previous versions of IGMP the leave and join are sent in two separate messages.
The multicast group that the STB joins contains streams with MPEG frames. There are different frames in MPEG, so-called I-frames which contain a full picture, P-frames contain incremental extrapolation information, and B-frames contain interpolation information. Since B and P frames depend on adjacent frames it is necessary that the STB receives a full I-frame before frames of a new channel can be decoded. This means that the average time for switching between channels will depend on the distance in time between I-frames. Typically for mpeg-2 the distance is around 0.5 seconds and for mpeg-4 part 10 it can be up to several seconds.
Other sources of channel switching delay relates to the buffer in the STB and network equipment, the time it takes for performing IGMP LEAVE/JOIN and other processing.
In order to alleviate the problem with channel switching delay there are various solutions existing today. One solution is to start a unicast session for the new channel to get the frames of the new channel down as fast as possible, and then performing a switch to the original multicast session when synchronization is possible. This solution implies overlapping unicast and multicast streams, since the unicast stream and the original multicast stream are transmitted simultaneously before the switch to the original multicast stream can be performed.
Overlapping unicast and multicast streams is an important limitation that typical fast channel change solutions based on unicast-based rapid acquisition of multicast streams techniques have because the overlapping streams require extra bandwidth. Even small overlaps of unicast and multicast sessions may severely impact the end-user experience, or significantly limit the efficiency of the network capacity utilization.